The present invention relates to a system for collecting and refining SF.sub.6 gas at maintenance and inspection of an insulated machine using SF.sub.6 gas as an insulating gas and a method therefor and, more particularly, to a system for collecting and refining SF.sub.6 gas and a method therefor suitable for recovering a deteriorated SF.sub.6 gas by collecting and refining.
Gas insulated machines commonly use SF.sub.6 gas which is an inflammable insulating gas. For example, in the past transformers have employed an insulating oil as the insulating material, but an inflammable insulating gas is used instead of the insulating oil because the insulating oil has a probability of occurrence of fire or explosion in a case of trouble. SF.sub.6 gas is widely used as the insulating gas. SF.sub.6 gas has a global warming property (hereinafter referred to as GWP) coefficient as large as approximately 25000 and accordingly has a large effect on global warming. Therefore, from the viewpoint of global warming prevention, it is preferable to prevent SF.sub.6 gas, even a small quantity, from discharging to atmosphere. Further, the density of SF.sub.6 gas is approximately five times as large as that of air, so there is a possibility that an accident can be caused resulting in injury or death by oxygen shortage when SF.sub.6 gas is discharged inside a room because it stagnates in the bottom portion of the room or the lowermost portion of the building.
Furthermore, it is known that SF.sub.6 gas is partly decomposed during use by arc discharge or the like, and part of the decomposed compositions react with moisture to produce acidic gases. Therefore, by-product refining gases are produced. That is, as shown in Table 1, SF.sub.6 gas is partly decomposed by discharges or arcing to produce acidic gases and metal fluorides such as HF, SOF.sub.2, SO.sub.2, H.sub.2 SO.sub.3, SF.sub.4, SF.sub.2, S.sub.2 F.sub.2 and so on.
TABLE 1 ______________________________________ SF.sub.6 .fwdarw. SF.sub.4, SF.sub.2, S.sub.2 F.sub.2, F.sub.2, Metal fluorides. When moisture exists, the following reactions occur. ______________________________________ SF.sub.4 + H.sub.2 O .fwdarw. SOF.sub.2 + 2HF SOF.sub.2 + H.sub.2 O .fwdarw. SO.sub.2 + 2HF SO.sub.0 + H.sub.2 O .fwdarw. H.sub.2 SO.sub.3 SO.sub.2 + F.sub.2 .fwdarw. SO.sub.2 F.sub.2 ______________________________________
In addition to these, there are some reports that CO.sub.2 and so on are produced from an organic component such as a winding portion, and that CF.sub.4 is partially produced. As described above, although SF.sub.6 gas is a useful gas as an insulating gas, SF.sub.6 gas has a large global warming property coefficient and accordingly release of SF.sub.6 gas to atmosphere is harmful to global warming, and SF.sub.6 gas may cause an accident such as oxygen shortage when SF.sub.6 gas is discharged inside a room because the density of SF.sub.6 gas is large, and in addition to these SF.sub.6 gas is harmful because it contains the acidic gases. Therefore, a method of collecting or recovering the deteriorated SF.sub.6 gas is also required in view of societal needs.
For the above reasons, at maintenance and inspection of an insulated machine using SF.sub.6 gas, a method of collecting and bringing back SF.sub.6 gas and refining the SF.sub.6 gas to be reused is proposed. For example, a decomposed gas treating system for a SF.sub.6 gas insulated electric machine that is disclosed in Japanese Patent Application Laid-Open No.9-856 proposes a method in which acidic gases among by-product gases contained in a deteriorated (used) SF.sub.6 gas is removed by bubbling in an alkaline solution and then the SF.sub.6 gas is filled in a compressed-gas cylinder to be brought back for refining the SF.sub.6 gas.
Further, an article entitled "1512 An SF.sub.6 Gas Collecting Apparatus in a Large Electric Power Test Site" presented at the National Meeting of the Institute of Electrical Engineers of Japan (1996) proposes a method in which SF.sub.6 gas is refined to a reusable purity by (1) removing metal fluorides, (2) removing decomposed gases, (3) removing moisture and (4) removing impurities using an air removing block. This method removes acidic gases using an alkaline solution, and proposes a system which has a dew-point meter and an oxygen concentration meter so that quantities of moisture and air content can be monitored. This process can satisfy the SF.sub.6 gas standard of "IEC-376 New Gas Standard" shown in Table 2.
TABLE 2 ______________________________________ Moisture lower than 15 wt ppm Air lower than 500 wt ppm Hydrolytic fluoride (HF) lower than 1.0 wt ppm ______________________________________
Although the need for methods in which deteriorated SF.sub.6 gas is collected at inspection and maintenance and refined to be reused is increasing as described above, the technologies disclosed in Japanese Patent Application Laid-Open No.9-856 and described in the article entitled "1512 An SF.sub.6 Gas Collecting Apparatus in a Large Electric Power Test Site" presented at the National Meeting of the Institute of Electrical Engineers of Japan (1996) are still technically insufficient to cope with the need. This is because the purity of the refined SF.sub.6 gas is still not sufficiently confirmed and guaranteed, and the system for collecting and refining SF.sub.6 gas is very large in size.
Initially, the confirmation and guarantee of gas purity will be described. SF.sub.6 gas is an important factor to in ensuring the insulation performance of a gas insulated transformer or a gas insulated switch-gear. Therefore, in order to refine and reuse the SF.sub.6 gas, it is required to confirm that the performance of the recovered SF.sub.6 gas is equivalent to that of new SF.sub.6 gas. If the recovered SF.sub.6 gas contains impurities and the insulating performance is degraded, the apparatus will be broken and damaged by occurrence of a short circuit inside the apparatus. Therefore, the collection and refinement requires confirmation by measuring the property of the recovered SF.sub.6 gas. Table 3 shows an analysis result of an SF.sub.6 gas used in a gas insulated switch-gear after repeating open and close switching. In order to confirm that the performance of the recovered SF.sub.6 gas is equivalent to that of new SF.sub.6 gas, it is necessary to measure the items and to confirm the concentrations. In order to do so, it is necessary to measure the items using measurement apparatuses matching the respective gases.
TABLE 3 ______________________________________ Example of a result of analyzing components of a deteriorated SF.sub.6 gas Concentration No. Item Contents (ppm) ______________________________________ 1 Solids Fluorides, Sulfides and others 2 Acidic gases SF.sub.4, SF.sub.2, S.sub.2 F.sub.2, SOF.sub.2, 5000 SO.sub.2 F.sub.2, HF and the like 3 Carbon dioxide CO.sub.2 1500 4 PFC CF.sub.4, C.sub.2 F.sub.6, C.sub.2 F.sub.4 and 1000 like 5 Air O.sub.2, N.sub.2 600 6 Moisture H.sub.2 O 20 ______________________________________
Further, since the gas insulated machine can be made small in size compared to that of the conventional method, the gas insulated machine in a substation in a town is usually installed in a basement from the viewpoint of effective use of the site area. In this case, since the gas insulating method has less fire potential compared to the conventional oil insulating method in addition to the advantage of small size, gas insulated machines are widely employed.
On the other hand, there are many substations of the ground installed type in the suburbs. In such cases, when the gas insulated machine capable of having a reduced installation area, it is advantageous.
Since the gas insulated machines are distributedly installed in many sites as described above, it is required that the system for collecting and refining the SF.sub.6 gas to reuse it is small and portable. If the system for collecting and refining is not small and portable, it is required to fill the collected gas into a compressed gas-cylinder and to transfer it to a place where a refining apparatus is installed and then after refining to transfer it to the site again. Therefore, the time period of the inspection process is lengthened and substantial work is required for carrying in and out the compressed-gas cylinder. In a case where new SF.sub.6 gas is filled after a deteriorated SF.sub.6 gas is collected, the delay in time period of the inspection process can be avoided, but the work for carrying in and out of the compressed-gas cylinder of the collected gas and the compressed-gas cylinder of new SF.sub.6 gas is still required. In addition to this, the treatment of the collected deteriorated SF.sub.6 gas is required. In the case of a basement substation, the amount of work required for the carrying in and out of the collecting and refining system and of the compressed-gas cylinders becomes large and the size of the transportation port also becomes a limitation. Therefore, a small sized collecting and refining system is required.
Both of Japanese Patent Application Laid-Open No.9-856 and the article entitled "1512 An SF.sub.6 Gas Collecting Apparatus in a Large Electric Power Test Site" presented at the National Meeting of the Institute of Electrical Engineers of Japan (1996) propose a wet method of refining deteriorated SF.sub.6 gas where acidic gases are removed by passing the deteriorated SF.sub.6 gas through lime-water solving calcium hydroxide in saturation. However, since the solubility of calcium hydroxide is small (0.16 g/100 g water at 20.degree. C.) and accordingly the amount of the solution becomes large when it is used in an aqueous solution, there are problems in that the size of the apparatus becomes large and that a large amount of waste water is produced and a lot of time and effort are required in the transportation and treatment of the waste water. Particularly, since the waste water has a potential of leakage, the waste water needs to be carefully handled and managed, and there are various kinds of problems in handling the waste water in a limited place such as a basement.
Since the conventional SF.sub.6 gas collecting system has only a gas collecting function but not a gas refining function, the conventional SF.sub.6 gas collecting system is designed based on the viewpoint of how fast SF.sub.6 gas is collected from a gas insulated machine in order to improve work efficiency. When SF.sub.6 gas is filled in a gas insulated machine, for example, at a pressure of 5 kg/cm.sup.2, the SF.sub.6 gas is firstly collected into a collecting tank until the pressure of the collecting tank and the pressure of the gas insulated machine reaches equilibrium by opening a communicating pipe line, and then the remaining SF.sub.6 gas is collected into the collecting tank using a compressor or a vacuum pump.
In a case of employing this method, the pressure at the machine side is decreased from 5 kg/cm.sup.2 to approximately 3 kg/cm.sup.2 within 1 to 2 minutes after opening the communicating pipe line. Then, the remaining SF.sub.6 gas is collected using the compressor. However, approximately 1/3 of the gas has been collected by the opening of the communicating pipe line. Since the efficiency of the compressor decreases as the pressure in the machine side is decreased, the amount of collected gas per unit time is decreased as the compressor is operated and the amount of collected gas per unit time reaches the minimum at the end of the collecting work.
In a case where the processing rate of gas per unit time in the refining process is varied regardless of whether the process is of a wet method or a dry method, the refining process system needs to be designed so as to meet the maximum processing gas flow rate. In a case where the concentration of acidic gases in the SF.sub.6 gas is reduced down to a predetermined value, design of the system of the wet method is performed by determining a used amount of alkaline solution to a unit volume of the treated gas (ratio of solution to gas) to a predetermined value. On the other hand, in a case of the dry method, the design is also performed by determining a ratio of the treated gas volume treated in unit time to a volume of an adsorption removing agent (SV value). Therefore, the system needs to be designed so as to meet the maximum gas flow rate when the processed gas rate varies, which leads to problems in that the system becomes large in size and consequently optimization and small-sizing of the system cannot be attained. Further, when the gas flow rate exceeds the value of design condition, a large amount of the alkaline solution is carried over by the gas in a case of the wet method to cause a trouble in the following process. Furthermore, regardless of the dry method or the wet method, there occurs a problem in that the initial removing performance cannot be attained.